T-wave alternans are characterized by a pattern of alternations in the amplitude of the T-wave component of an ECG, where the even beats systematically display a different amplitude than the odd beats (an “ABABAB . . . ” pattern of beat signatures). Many prior research efforts have found correlations between the amplitude of the T-wave alternans during periods of increased heart rates, and sudden cardiac arrest or arrhythmias. Verrier and Cohen, in their chapter “Risk Identification by Noninvasive Markers of Cardiac Vulnerability” (Foundations of Cardiac Arrhythmias, Spooner and Rosen editors, Marcel Dekker, Inc., 2000), provide an overview of past research and describe a signal processing method for determining the presence of microvolt level alternans. Summarily stated, the ECG signal is evaluated to identify sequential data points within the T-waves. The amplitude of these selected points, from successive beats, forms pseudo time series that are next subjected to Fourier analysis to create a power spectrum; the power at the Nyquist frequency of this spectrum provides an estimate of the energy of the beat-to-beat fluctuations in the amplitude of the T-wave. The power spectra from successive individual spectra associated with different offset times within the T-wave coda are averaged to establish a composite power spectra, which is claimed to be useful in assessing patient risk for sudden cardiac arrest or arrhythmias. Clinical observations and trials have shown that persons who exhibit T-wave alternans at relatively low heart rates, i.e., ˜110 bpm, are at greater risk of developing fatal arrhythmias than those who exhibit alternans at heart rates approaching their maximum target heart rate. Both this publication, as well as U.S. Pat. Nos. 4,802,491; 5,148,812; and 5,713,367 relating to this and related approaches are incorporated herein by reference.
While the above-described method may be valuable for establishing the gross existence and severity of T-wave alternans, the analysis is limited to only the average amplitude of the alternan signal across the entire T-wave signal. Clinical experience with stratifying patient risk of sudden cardiac death based upon this simplistic characterization of the alternan signal are typified by a high rate of indeterminacy—typically as high as 30% of the patient tests for alternans are indeterminate.